Under the action of good wheels and a level track or bridge system, the distribution of train wheel loads on the concrete ties, according to the conventional wisdom, depends on:
(a) the tie spacing; PA1 (b) the ballast stiffness or the stiffness of the tie-girder bearing pads in the case of open deck bridges; and PA1 (c) the size of the rail. Changing the size of the tie and the characteristics of rail-tie pad has not generally been thought to have a significant effect on the distribution of train wheel load on concrete ties. This invention concerns improvements to the pads in ways not previously perceived as being available.
In practice, track and bridge ties are subjected to moving axle loads. Because of the vehicle speed, wheel imperfections and random differences of levels and other differences in the field, the dynamic load transmitted to the concrete tie is much higher than the static load. This increase over the static load manifested itself in 1980 along the North-East rail corridor (between Washington D.C. and Boston) where concrete track ties were found to have developed hairline cracks only a few months after their installation. It should be noted that concrete ties normally are thought to have a projected life expectancy of 50 years. Similar experiences of tie failure have been reported by the Canadian, European and Japanese railways.
To accommodate this increase of dynamic loads over the static load and the resulting risk of damage, the code committees in various countries use the so called "Impact Factor", (I.F.), in concrete tie design to accommodate for the dynamic component of the railway track loading. In North America, an Impact Factor of 60% (excess design load over 100% static load capacity) was initially recommended by the Association of American Railroads (AAR). The disappointing performance of concrete ties designed with the 60% increase factor led to a recommendation by the AAR for an "Impact Factor" (I.F.) of 150% which is presently used today. Yet concrete ties designed with the 150% Impact Factor have suffered the same fate as their predecessors. Presently, a new proposal has been tabled by some members of the AAR asking for an increase of the Impact Factor to 200%.
To understand the nature of distribution and attenuation of dynamic (especially impact) loading, attention must be paid to the effects of rail-to-tie pad stiffness and tie-to-girder pad stiffnesses.
It has been found that the dynamic over-loading of concrete ties is not influenced by the train speed, provided that the train wheels are smooth and have no surface irregularities, such as "shells" or flats. When these are present on the wheel running surface, the response of the concrete tie to the wheel loading has been observed to be dependent on the train speed and the impact load is dependent on the unsprung mass of the train-wheel set. At low speeds (0-40 mph), (0-64 km/h), there can be a complete unloading of the ties followed by impact. At high speeds (above 50 mph {80 km/h}), particularly in the case of lighter passenger trains, the wheels can become temporarily airborne for a very small time interval, and then impact on the rail a number of times on landing. This creates very high dynamic loads not only on the supporting tie, but also on other track and vehicle components.
To protect concrete ties and to reduce the probability of rail or wheel fractures or shelling due to the impact resulting from the wheel defects on the various trains, the EVA (Ethyl Vinyl Acetate) pad, a solid and very stiff (stiffness=10800 kips/in) pad, was developed by Pandrol Limited in Britain. This pad has been used extensively between rails and concrete ties. Research findings have shown, however, that solid pads and other equivalently stiff pads transmit enough impact energy to cause cracking of concrete ties. Solid, stiff pad designs commercially available do not afford the degree of protection for ties that would be desired by the railways. As indicated previously, in some cases, the concrete ties have developed cracks less than six months after being put into service.
Attempts in the past to improve the performance of the tie-pads have included the selection of certain surface profiles, such as linear grooves, perforations, surface patterns in the form of directly opposed studs and shallow dimples.
Prior patents that have addressed these issues are as follows:
U.S. Pat. No. 2,656,116--Protzeller assigned to Arthur Wm. Nelson (perforations)
U.S. Pat. No. 4,254,908--Matsubara assigned to Tokai Rubber Industries Ltd. (offset grooves)
U.K. 2,161,524--Brister et al, issued to Pandrol Limited (opposed studs)
U.S. Pat. No. 4,648,554--McQueen, issued to Acme Plastics Inc. (offset dimples)
The effect of such profile variants has been to provide pads that substantially absorb applied loads by undergoing compression. Design control over the response of such pads under compression is, however, limited.
Ideally, a railway tie pad should be capable of both absorbing the equivalent static load of a heavy, slow-moving freight train, and the dynamic, high frequency, shock loading created by higher speed trains. Such dual characteristics are not easily found in a single pad design.
This invention achieves an improvement in the design for the rail-tie pads by controlling the stiffness of the pad under such variable conditions. This is done by modifying its shape in order to improve the attenuation of impact loading. Tie pads made in accordance with the invention rely on the creation of shear stress within the pad and/or novel surface profiles to provide a means for creating a multi-stage response function that is suitable for sustaining both light and heavy loads and, at the same time, attenuating high frequency dynamic stresses.
These and further features of the invention will be apparent from the description which now follows.